Promethium at a glance
- Atomic number
- 61
- Symbol
- Pm
- Atomic mass
- 145 u
- Category
- Lanthanide
- Period
- 6
- Electron configuration
- [Xe] 4f 5 6s 2
- State at 25 °C
- Solid
- Melting point
- 1042 °C
- Boiling point
- 3000 °C
- Density
- 7.26 g/cm³
- Electronegativity
- 1.13
- 1st ionization energy
- 540 kJ/mol
- Oxidation states
- +3
- Discovered
- 1945
Atomic structure
A neutral promethium atom has 61 protons and 61 electrons, and its most common isotope carries about 84 neutrons. One atom weighs about 2.41 × 10⁻²⁵ kg, so a mole of promethium (6.022 × 10²³ atoms) weighs 145 g. That nucleus holds 1.38 neutrons for every proton. The electrons fill 6 shells (K 2, L 8, M 18, N 23, O 8, P 2), which gives the configuration [Xe] 4f 5 6s 2. As a f-block element, both its outer s electrons and its inner f electrons can take part in bonding, which is why it shows more than one oxidation state. All of its isotopes are radioactive, so any promethium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) promethium is a solid. It melts at 1042 °C and boils at 3000 °C, so it stays liquid across a 1958-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 7.26 g/cm³, 7.26 times as dense as water. Its atoms have a radius of about 185 pm (Neodymium next door is 185 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.13 on the Pauling scale. Pulling off its outermost electron takes 540 kJ/mol. A bond between promethium and chlorine would be mostly ionic, with an electronegativity difference of 2. Its common oxidation states are +3, so in compounds you will usually meet it as Pm³⁺. It shares much of its chemistry with the other lanthanides, especially its neighbours Praseodymium, Neodymium, Samarium and Europium. Handle with care: as a pure element it is radioactive.
Where it comes from
It is vanishingly rare in nature, existing only briefly as part of radioactive decay chains. It was first identified in 1945, making it the 96th element to be discovered (counting the 11 known since antiquity together).